A sapphire/gallium nitride laminate is a useful substrate for a light emitting device (e.g., LED, LD) or other electronic devices, and it has been conventionally prepared by growing gallium nitride on a sapphire substrate by metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), etc. Such a laminate, however, suffers from the problem of bending deformation due to the interfacial strain caused by the differences in the lattice parameter and thermal expansion coefficient between sapphire and gallium nitride, as shown in FIG. 1 (11: sapphire, 12: gallium nitride). The degree of such bending deformation of a sapphire/gallium nitride laminate increases as the thickness of the gallium nitride (GaN) film formed on sapphire increases, adversely affecting the quality and productivity of electronic devices comprising the laminate.
To solve the problem of bending deformation, there have been developed various methods of controlling the GaN/sapphire laminate interfacial state, e.g., a low temperature GaN buffer growth method (U.S. Pat. No. 5,290,393), a PENDEO method using artificially prepared pores (U.S. Pat. No. 6,177,688), an AIN buffer growth method (Applied Physics Letter, Vol. 53, p185, 1988), and an AIN embossing growth method (U.S. Pat. No. 6,528,394). Such interface controlling methods may be applied to the growth of a few to a several tens μm thickness film, but they are not suitable for growing a thick GaN film of several hundred μm thickness due to the generation of cracks (13) on the film influenced by residual strain, as shown in FIG. 2.
In order to inhibit the formation of cracks, EP 1 946 718 and 1 116 827 suggested a method comprising the steps of growing a GaN film (31) on a sapphire substrate (11) to a thickness of several ten or hundred μm, separating the grown film (31) from the substrate so as to remove strain therefrom, and then further growing a separate GaN layer (32) on the separated GaN film, as shown in FIG. 3. However, this method requires the complicated step of removing the substrate, and there still remains some strain on the separated film, which leads to the generation of generating bending deformation or cracks in the grown GaN layer.
Also disclosed in J. of Crystal Growth, High-quality and crack-free GaN films grown on cracked Si-doped GaN templates, accepted in 2001, M. Hao et al., is a method of growing a GaN film having little cracks; comprising growing a Si-doped GaN film on a sapphire substrate to a thickness of several μm by metal organic chemical vapor deposition (MOCVD) to induce crack formation on the surface of the GaN film, and then growing a pure GaN layer thereon. However, this MOCVD method has the disadvantage that the film growth rate is too low to obtain a film having a thickness over several hundred μm. Also the interfacial stress derived from the differences in the thermal expansion coefficient and lattice constant between the sapphire substrate and the Si-doped GaN film is not great enough to propagate the cracks formed on the Si-doped GaN layer to the bottom of the sapphire substrate, and therefore, it is difficult to grow a GaN film having a thickness greater than 1 mm.